Subsea control systems and apparatus

ABSTRACT

A subsea actuator for operating a valve (20) or the like comprises a movable wall member (19) fixed to an axially slidable stem and separating a pressure chamber (2) from a space (3) which is subjected to the hydrostatic pressure of ambient seawater. A selector valve (11) is operable to connect the chamber (2) to a source of pressurised fluid, e.g. a source of pressurised gas (14), or to a drain e.g. connected to atmosphere by a pressure vessel (12) and pipe (15). A blowdown valve (16) and dump valve (13) are provided for ejecting liquid collected in the vessel (12). The space (3) may be connected to an open-bottomed container (17) to provide a gas barrier between the seawater and the interior of the actuator. Supply of pressurised gas to the chamber (2) causes the member (19) to be driven forwards, gas being allowed to flow past the member into the space (3) during this movement to assist the expulsion of water from the container (17) and the expulsion of any contaminants from within the actuator. When the chamber (2) is connected to atmospheric pressure the hydrostatic pressure of the seawater causes the member (19) to be driven in the reverse direction.

This invention relates to control systems and apparatus for opening andclosing valves on subsea installations associated with oil and gasproduction from subsea locations.

The Prior Art

Hitherto, subsea valves have been operated manually by divers, poweroperated by manned or unmanned submersible vehicles, or remotelyactuated by means of integral valve actuators and control systemsutilising mineral oil or specially formulated water-based solutions asthe power fluid.

The remotely actuated systems are to a large extent versions ofconventional surface equipment adapted for marine use and they have thedisadvantage that to provide reliable operation in an environment ofcorrosive seawater which contains particulate matter and fostersbiological activity, it is necessary to isolate internal components fromseawater and utilise specialised power fluids with correct levels ofadditives. These power fluids tend to be expensive and the additives, orbase constituents, often are environmentally deleterious. A furtherdisadvantage of existing systems is the need to supply, or resupply,suitable power fluids. These drawbacks have inhibited the development ofsubsea closed loop control systems.

SUMMARY OF THE INVENTION

The aim of the present invention is to eliminate, or at leastsubstantially alleviate the drawbacks of the prior art subsea valveactuators, and in accordance with the invention there is provided asubsea actuator for operating a subsea component such as a valve orsimilar linearly operated device, comprising a housing, a movable wallmember cooperating with the housing to confine therewith a substantiallyclosed chamber separated by the movable wall member from another fluidspace, the wall member being fastened to an elongate output member andbeing movable under forces acting against the opposite sides thereof todisplace the output member longitudinally, and inlet means to connectthe chamber to either a source of pressurised fluid at a pressuregreater than the hydrostatic pressure of the ambient seawater, or todrain outlet at a pressure not greater than the hydrostatic pressure ofthe ambient seawater, the actuator being arranged for the other space tobe at the hydrostatic pressure of ambient seawater and the movable wallto be moved in a forward direction when the chamber is connected to thesource of pressure fluid and to be moved in a reverse direction when thechamber is connected to the drain outlet.

A control system based on an actuator in accordance with the inventionmay use untreated seawater and a pressurised fluid preferably obtainedfrom a subsea source and possibly also untreated sea water as the powermediums, whereby the sea-bed hydrostatic pressure at least contributesto the production of a force acting on the movable wall member todisplace it, such as for actuating a valve. The pressurised fluid may bea low density fluid (gas), seawater pumped to a pressure above theambient hydrostatic pressure of the actuator or could be taken from awell stream to which the valve being controlled is fitted. The controlsystem will include a valve for selectively connecting the chamber ofthe actuator to the source of pressurised fluid or to a drain. Whenseawater is utilised as the pressure fluid the drain can lead directlyto the surrounding seawater, but in this case the actuator will requirean additional component such as a spring for driving the movable wall toproduce a rearward stroke of the output member as the movable wall willbe exposed to the hydrostatic pressure of the ambient seawater on bothsides. The pressurised fluid can by a low density fluid, includinggases. If gas is used as the pressurised fluid, the drain may be led toa level above the sea surface, preferably via a closed pressure chamberto accelerate actuator operation, so that the seawater pressure in theother space may be solely responsible for the rearward displacement ofthe movable wall member when the chamber is connected to the drain.

The other space, which is preferably another chamber in the housing, maybe arranged to be flooded with seawater, but in an alternativeembodiment the actuator is equipped with means to provide a gas barrierbetween the interior of the actuator and the surrounding sea, which canhelp minimise corrosion and biological activity and may in additionprovide a visual indication of faults occurring or developing in thesystem. The means providing the gas barrier may be a container connectedto an outlet orifice of the actuator at the upper end of the containerand open to the sea at the lower end, the container being of greatervolume than the total swept volume of the actuating actuator, and thegas trapped in the container forming a fluid barrier between the systeminternals and the sea, due to the different densities of the operatinggas and seawater.

The component parts of the actuator according to the invention, and theother devices included in the subsea control system, will be constructedand manufactured from suitable materials consistent with exposure tountreated seawater and the subsea environment. The movable wall of theactuator may be constructed to provide a leakage flow from one chamberto the other chamber during movement of the wall member from a rearmostposition to a forwardmost position, which can secure the advantage offlowby deterring accumulation of biological and other deposits withinthe actuator.

In one of the preferred embodiments of the invention it is preferredthat the system operates with compressed gas in contact with the systeminternals, but inadvertent flooding of the control system with seawater(always a possibility due to damage) will not render the systeminoperative as provision is made for the ejection of unwanted fluids andgas flushing through gas being allowed to flow past the movable wallmember.

It will be understood that the actuator of the invention constitutes athrusting device which is mounted on or adjacent to a process valve orsimilar mechanism being controlled. The movable wall member forms athrust-producing member which is attached to an output member,conveniently an axially slidable stem. The actuator housing and movablewall member define a pressure containment means such that when apressure higher than the seabed ambient is applied, the device willstroke in one direction, referred to as the forward direction,displacing fluid on the other side of the thrust-producing member indoing so, and when pressure not greater and preferably lower than theseabed ambient is applied, the device will stroke in the other, rearwarddirection under the influence of the higher surrounding hydrostaticpressure, possibly aided by a spring force.

A lower operating pressure less than seabed ambient may be obtained byconnecting the internal volume of the thrusting device, via a selectorvalve, to a conduit or pressure vessel maintained at or near toatmospheric pressure by direct conduit connection to a point above thesea surface. The higher operating pressure may be obtained from asurface installation or subsea source connected to the selector valve.

A conduit or pressure vessel, within which solenoid or pilot valves canbe contained and which is connected to the surface installation can beprovided with a non-return dump valve to enable any collected fluids tobe ejected to the sea when the conduit or vessel is temporarilypressurised above the surrounding seabed hydrostatic pressure. Thisdevice enables the control system to be kept serviceable irrespective ofseawater ingress into the system.

With a control system as described herein a switching device at thecontrol point enables the process valve to be opened or closed whenrequired.

Some specific embodiments of the invention will now be described indetail, by way of example, with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical schematic layout of the overall control system;

FIG. 2 illustrates in axial cross section a piston-type actuator orthrusting device;

FIG. 3 illustrates in axial cross section a diaphragm type thrustingdevice;

FIG. 4 illustrates in axial cross section a bellows type thrustingdevice;

FIG. 5 illustrates in axial cross section an alternative piston typethrusting device; and

FIG. 6 shows a schematic layout of a pressure vessel containing one ormore control valves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is shown a subsea valve control system, the principalcomponents of which are a thrusting device (valve actuator) 10, a fluidor, as shown, electrically-operated selector valve 11, a pressure vessel12, a non-return dump valve 13, a source 14 of high pressure low densityfluid which in the particular example is gas, a connecting pipe 15leading to the surface, a surface mounted blowdown selector valve 16, abarrier container 17, and a switching device 18.

The system is shown in the non-operated, or fail safe condition. Thepressure vessel 12 is at approximately atmospheric pressure (14.7psi+air pressure head due to water depth) due to the upper end of pipe15 being connected to atmosphere by the valve 16.

The valve actuator 10 has a housing 1 accommodating a movable wallmember (shown as a piston 19 in FIG. 1) separating first and secondchambers 2, 3. The housing defines a port opening into the first chamber2 and which is connected to a control port of the selector valve 11which is operable to connect the first chamber 2 to either the source ofpressure fluid 14 or, as in the illustrated condition of the system, tothe interior of the pressure vessel 12. The second chamber 3 has a port23 connected to the top of the container 17, the lower end of which isopen so that chamber 3 is subject to the hydrostatic pressure of theambient seawater i.e., the chamber 3 is in direct fluid communicationwith the ambient seawater. The volume V2 of the container 17 is greaterthan the volume V1 of the second chamber 3 so that the trapped gasvolume prevents seawater entering the actuator during normal operationthereof. The piston 19 is fixed on the end of an axial stem or pistonrod which is coupled to the operating member of the process valve 20being controlled. The piston 19 is shown to be equipped with seals 21for cooperation with internal surfaces of the housing.

The piston 19 is driven to the right under the influence of the trappedlow density fluid (gas) at seawater pressure in chamber 3 and holds theproduct valve 20 in the closed position. The piston 19 presses the endstop abutment seal 21 against the confronting wall of the housing,thereby sealing off the pressure vessel 12 and preventing entry ofsurrounding seawater into the system via the barrier container 17.

When the selector valve 11 is operated, the pressure vessel 12 is sealedoff from the chamber 2 and high pressure gas is admitted to the valveactuator 10, to stroke product valve 20 to the open position. Leakage ofgas past the piston during its forward stroke provides gas flushing ofthe cylinder during the working stroke and raises the pressure in thedischarge end, i.e. chamber 3, of the actuator to a level higher thanthe surrounding hydrostatic pressure, thereby forcing the seawater levelin container 17 down until the gas can bubble freely, to the surface.The flowby gas therefore effectively maintains a gas seal between thesea and the system internals. When the actuator strokes in the oppositedirection, the seawater level will rise within container 17, but willnot enter the cylinder or control system. Seawater can initially beprevented from entering the actuator by fitting a blow off cap 25 whichis automatically jettisoned upon pressurising the actuator and hence thecontainer. The piston 19, at the end of its forward stroke is drivenagainst the abutment seal 22 at the opposite end of the cylinder toprevent continuous gas leakage to sea via the barrier container 17.

Should any seawater accumulate in the bottom of the pressure vessel 12,this can be ejected directly to sea via a drain fitted with a non-returnvalve 13, by periodically pressurising the vessel 12, above thesurrounding seawater hydrostatic pressure by operating blowdown valve 16to connect the upper end of pipe 15 to a suitable source of gaspressure.

To shut the product valve 20, selector valve 11 is operated by switch18, so that the gas supply 14 is isolated and the working chamber 2 ofvalve actuator 19 is exhausted to the surface via the pressure vessel 12and pipe 15. The capacity of the pressure vessel 12 allows the valve toshut at a higher rate than the gas is exhausted to the surface, but thevessel is not essential and the drain port of the selector valve couldbe connected directly to the surface by a conduit such as pipe 15. Therate of movement of the piston is also assisted by the flowby featurewhich allows the piston to move through the exhausting fluid.

FIG. 2 illustrates the principal features of a piston-type thrustingdevice (Valve Actuator) 10.

To enable the valve actuator to operate satisfactorily with untreatedseawater present, the design of the actuator and the control systemenable high flowby rates to provide efficient gas purging of theactuator to flush out contaminants and minimise internal biologicalgrowth. During forward movement of the piston (to the left as depictedin the drawing), fluid flow passes reverse fitted seal 25 or non-returnvalve 26 and is directed through a duct 27 provided in the piston nearthe bottom edge thereof to disturb and scavenge particulate matter onthe lower internal surface 28 of the cylinder. The seal 25 and/ornon-return valve 26 prevents gas flow past the piston during its reversestroke. High bypass rates are also conducive to large operatingclearances between the piston and cylinder wall thereby minimisingpotential seizure problems. The cylinder or housing of the actuator isshown to be formed by two end walls and a frame.

The materials of construction of the actuator will be plastic orcomposite materials and/or alloyed metals to minimise corrosive effectsof direct seawater contact. The use of composite materials forconstruction can help to minimise marine biological growth asanti-biological inhibitors may be mixed with the composite materials.

FIG. 3 illustrates an alternative design of subsea actuator utilising adiaphragm 29 clamped between a pair of support plates on the piston rodor stem 33 as the thrusting member or movable wall member. Thisconstruction eliminates any sliding components within the actuator andprovides a substantially frictionless arrangement. High flowby rates areachieved by a duct 30 formed in the stem 33 which provides flow tochamber 3 on the discharge side of the diaphragm via an annulus 32around the actuator stem 33 and valve seats 34 and 35 on a seat plate36. Corresponding seats are provided on the actuator stem 33 forcooperation with the seats 34, 35 respectively in the end positions ofthe stroke, so that the valve seats prevent flowby or sea return at theextreme positions of actuator travel. High pressure gas is connected tosupply port 37 to operate the actuator whilst gas at hydrostatic sea bedpressure is connected to port 23 to provide the return (and "fail safe")force via barrier container 17. All other features of the subseaactuator construction regarding materials etc., will be similar to thepiston type actuator described in FIG. 2, with the exception of theelastomer diaphragm 29.

FIG. 4 shows an alternative bellows-type actuator comprising a bellows38 supported by internal rings 39 to prevent internal collapse andexternal rings 40 to prevent outward bursting. The bellows is sealed atone end to a plate 41 forming a stationary housing wall. The other endof the bellows 38 is sealed to the periphery of a plate 42 fixed to thepiston rod or stem 33 and constituting a movable wall or thrustingmember. When high pressure gas is connected to port 43 opening intochamber 2 within the bellows, the bellows expands axially, the plate 42moving to the right as seen in the drawing, thereby to open the processvalve 20 (not shown). Expansion of the bellows displaces gas from anopened-bottom barrier canopy 44 within which the actuator is housed andhence lowers the sea level within said canopy. During this forwardworking stroke, flowby gas passes into canopy 44 from chamber 3 via aport 45, such communication being interrupted at either end of theworking stroke by seats 46 and 47 engaging with complementary seats onthe stem 33 in a manner similar to the diaphragm actuator shown in FIG.3. The canopy is suitably sized to ensure the sea level is maintainedbelow the contact level of the actuator under all conditions.

When the port 43 is connected to atmospheric pressure by operation ofthe selector valve, the actuator strokes in the reverse direction to theposition shown in the drawing under the pressure, namely the hydrostaticpressure of the ambient sea water, acting on the outer face of plate 42.

FIG. 5 shows an alternative piston actuator arrangement wherein a highflowby rate is achieved by a seal-less piston 48. Optionally a seal ring49 integral with the piston engages the cylinder end wall 50 to preventflowby at one end of the piston stroke. At the other end, a seat 51 onthe piston contacts a seat 52 situated on an inwardly directed lip atthe otherwise open end of the cylinder. This arrangement minimises thecorners and crevices in which particulate matter may accumulate in thefunctioning parts of the actuator. An outer case 54 is provided tocollect and transfer gas to a barrier container 17 via a port 23 aspreviously described. Alternatively, in place of the case 54 a coarsefilter screen may be applied so that the chamber 3 is in directcommunication with the ambient seawater.

The materials of construction would be similar to piston type actuatorpreviously described and shown in FIG. 2.

FIG. 6 shows a schematic arrangement of the pressure vessel containingone or more selector valves 11, for the operation of one or more productvalves 20. The selector valves (for multiple systems) would comprise acommon manifold 55, mounted and connected to a common pressurised gassupply 14, and would exhaust into the same pressure vessel 12.

The pressure vessel could be arranged for modular replacement formaintenance although it is not envisaged that it will be necessary withselector valves 11, specifically designed for the system conditions.

The control system and actuators specifically described hereinabove havepressurised gas as the power medium. However, other sources of fluidunder pressure may be used and in particular local sources ofpressurised fluid available at the seabed could be utilised. Thus, thewell stream 70 to which the process valve 20 is fitted energy of thecould be employed, as indicated by a connecting pipe 71 depicted inbroken line in FIG. 1. In addition, seawater raised to a suitableoperating pressure by a pump 75 (FIG. 1) mounted at the seabed can beused as the power medium. If seawater is used as the power medium,certain modifications will be appropriate to the system and theactuators disclosed. Thus, the pressure vessel 12 and pipe 15 may beomitted, the selector valve then being arranged for connecting thechamber 2 of the actuator to a drain leading directly into the sea. Inorder to provide a return force on the movable wall member when it issubjected on both sides to the hydrostatic pressure of ambient seawater,a spring, such as the coil spring as shown for exemplary purposes inFIG. 5, may be included within the actuator. Of course it will beappreciated that the actuators shown in the other drawings can beequipped with equivalent return springs.

In addition, when operating with raw sea water, the barrier container 17is obviated and the actuator may be constructed so that chamber 3 opensdirectly to the ambient seawater, which can help to facilitate theexpulsion of foreign matter from within the actuator.

I claim:
 1. A subsea actuator for operating a subsea component such as avalve, comprising a housing, a movable wall member cooperating with thehousing to confine therewith a substantially closed chamber separated bythe movable wall member from another fluid space, the wall member beingfastened to an elongate output member and being movable under forcesacting against opposite sides thereof to displace the output memberlongitudinally, and inlet means to connect said chamber to either asource of pressurised fluid at a pressure greater than the hydrostaticpressure of the ambient seawater, or to a drain outlet at a pressure notgreater than the hydrostatic pressure of the ambient seawater, saidactuator being arranged for said other space to be in direct fluidcommunication with and thereby at the hydrostatic pressure of ambientseawater and said movable wall to be moved in a forward direction whenthe chamber is connected to the source of pressure fluid and to be movedin a reverse direction when the chamber is connected to the drainoutlet.
 2. An actuator according to claim 1, wherein the other space isanother chamber defined within said housing.
 3. An actuator according toclaim 1, wherein the wall member is movable between opposed forward andrear end positions defined by stop faces fixed relative to the housing.4. An actuator according to claim 3, wherein means is provided to allowpressurised fluid to pass from said chamber to said other space duringmovement thereof from said rear end position to said forward endposition.
 5. An actuator according to claim 1, wherein the wall memberis a piston and the housing defines a cylinder accommodating the piston.6. An actuator according to claim 4, wherein the wall member is apiston, the housing defines a cylinder accommodating the piston, andsaid means comprises a clearance between the piston and cylinder.
 7. Anactuator according to claim 4, wherein the wall member is a piston, thehousing defines a cylinder accommodating the piston, and said meanscomprises a duct in the piston and a one-way flow device to permit flowthrough the duct from said chamber.
 8. An actuator according to claim 1,wherein the wall member comprises a diaphragm having an outer peripherysealed to the housing.
 9. An actuator according to claim 4, wherein thewall member comprises a diaphragm having an outer periphery sealed tothe housing and said means comprises a duct formed in the output member.10. An actuator according to claim 1, wherein the movable wall member issealed to one end of a bellows, the other end of the bellows is sealedto a stationary housing wall, and said chamber (2) is confined withinsaid bellows.
 11. An actuator according to claim 4, wherein the movablewall member is sealed to one end of a bellows, the other end of thebellows is sealed to a stationary housing wall, said chamber is confinedwithin said bellows, and said means comprise a passage defined betweensaid housing wall and said output member.
 12. An actuator according toclaim 6, wherein at least one of said stop faces defines a seal forclosing off communication between said chamber and said other space (3)in the forward end position of the wall member.
 13. An actuatoraccording to claim 6, wherein the output member or wall member isprovided with opposed sealing faces for cooperation with said stop facesto interrupt communication between the chamber and space (3) in therespective end positions of the wall member.
 14. An actuator accordingto claim 1, in combination with valve means connected to the inlet meansand operable to connect the chamber to a source of seawater underpressure or to a drain outlet opening into the seawater.
 15. An actuatoraccording to claim 1, in combination with first valve means connected tothe inlet means and operable to connect the chamber to a natural sourceof fluid under pressure or to a drain outlet.
 16. An actuator accordingto claim 1, in combination with first valve means connected to the inletmeans and operable to connect the chamber to a source of pressurized gas(14) or to a conduit connected to atmospheric pressure.
 17. An actuatoraccording to claim 16, wherein the conduit includes a non-return dumpvalve controlling an outlet opening into the sea, and the upper end ofthe conduit is connected to a second valve means operable to connect theconduit to a source of gas under pressure for pressurising the conduitto discharge liquid from the conduit.
 18. An actuator according to claim17, wherein the first valve means has a drain outlet connected to apressure vessel, and the pressure vessel is provided with the outlet anddump valve.
 19. An actuator according to claim 1, including a hollowopen bottomed container (17) connected to the actuator for trapping avolume of gas therein, and said other space of the actuator is in directfluid communication with ambient seawater through the container, thehydrostatic pressure of the ambient seawater being transmitted to saidspace by the gas trapped in said container.
 20. An actuator according toclaim 19, including a closure for temporarily closing the containerbottom, said closure being removable by pressurisation of the containervia the actuator.
 21. An actuator according to claim 19, wherein saidother space is enclosed within the housing and connected through a portto the container, and the volume of the container is greater than thedifference between the maximum and minimum volumes of said other space.